Air bags of occupant protection systems are expensive and in certain circumstances are dangerous. It is therefore desirable to avoid deployment when the seat is empty to save the cost of replacement. It is desired to avoid deployment when circumstances do not warrant deployment or when deployment might do more harm than good. It is particularly important to deploy the airbag judiciously when the seat is occupied by a child or by a very small adult. A system is desired to reliably distinguish an adult from a child even when the child is in a child seat and belts retaining the child seat are under substantial tension.
Occupant protection systems typically include a "sensor and diagnostic module" or "SDM" which senses the severity of a vehicle crash, monitors elements of the occupant protection system for proper operation, and deploys occupant protection devices. SDMs typically include a microprocessor, an accelerometer, an arming sensor, circuitry interconnecting the aforementioned components and switches for initiating deployment of the occupant protection devices. SDMs may be connected for receiving input from other sensors responsive to aspects of the occupancy of the seat.
To optimally deploy an airbag the SDM must take into account the weight of a seat occupant. Seat occupant weight sensors sense the weight of the occupant and communicate that weight to the SDM. With certain known seat occupant weight sensing systems seat belt tension affects the weight measurement therefore, for those systems, seat belt tension must be measured and communicated to a microprocessor of the SDM.
Capacitance sensing semiconductors are made by Quantum Research Group of Pittsburgh, Pa. and others. These devices sense small capacitances and certain of the designs provide output that varies linearly or monotonically with the capacitance being sensed.
A seat belt tension sensor must meet certain requirements: For accuracy and long life, friction in the mechanism must be minimized. The sensor must be accurate over a wide range of temperatures. The sensor must not rattle when the roads are rough. The seat belt tension sensor mechanism must withstand about one thousand pounds of seat belt force repeatedly without damage and not fracture or otherwise fail to restrain the occupant under about four thousand pounds of seat belt force, which could occur when the vehicle collides with an obstacle. No known design meets these requirements at a low cost.
Known force sensors must be protected from forces greatly in excess of the forces they are designed to measure. A seat belt tension sensor incorporating a known force sensor must protect the force sensor from the large forces that sometimes occur. Providing protection adds to the cost and complexity of the seat belt tension sensor. Accordingly, a force sensor that can measure forces on the order of thirty pounds while not being damaged by forces on the order of one thousand pounds is desired.
Of the known distance sensing means, capacitance sensing is advantageous for being inherently insensitive to temperature, not requiring permanent magnets, and being insensitive to the material used for sensing elements.
A general object of this invention is to provide a seat belt tension sensor offering low cost and superior performance which also overcomes certain disadvantages of the prior art.